Thermally developable diazotype printing paper and composition therefor

ABSTRACT

DIAZO COMPOSITIONS CONTAINING CRITICAL PROPORTIONS OF DICYANDIAMIDE AND THIOUREA AS THE THERMAL DEVELOPING AGENTS TOGETHER WITH ALKALI ACID SULFATE STABILIZER COMPOUNDS, FOR USE IN FORMING THERMALLY DEVELOPABLE PHOTO DIAZO PRINTING PAPER ARE DISCLOSED. THESE DIAZO PRINTING PAPERS ARE CHARACTERIZED BY GOOD IMAGE DENSITY, LONG SHELF-LIFE AND GOOD CHEMICAL STABILITY.

United States Patent Oflice 3,660,581 Patented May 2, 1972 3,660,581 THERMALLY DEVELOPABLE DIAZOTYPE PRINT- ING PAPER AND COMPOSITION THEREFOR Robert M. Levy, Kalamazoo, Mich., assignor to Allied Paper Incorporated, Kalamazoo, Mich. No Drawing. Filed Apr. 23, 1969, Ser. No. 818,824 Int. Cl. C03c U60, N58 US. Cl. 96-91 7 Claims ABSTRACT OF THE DISCLOSURE Diazo compositions containing critical proportions of dicyandiamide and thiourea as the thermal developing agents together with alkali acid sulfate stabilizer compounds, for use in forming thermally developable photo diazo printing paper are disclosed. These diazo printing papers are characterized by good image density, long shelf-life and good chemical stability.

This invention relates to improvements in thermally developable diazotype photoprinting materials. Record- 1ng papers known in the arts as diazotype papers are useful for recording optical images. These papers are basically of two modifications, the ammonia type and the moist type. The image reproduction is accomplished by exposing a sensitive stratum on a carrier support, usually paper, to actinic light. The portions thus exposed are deactivated in proportion to the intensity of the exposure. The remaining portions are developed to give a colored dye image under basic conditions such as by exposure to moist ammonia vapor or to an alkaline liquid developer.

Both of the above diazotype processes have disadvantages. In the vapor developing process a gas source is required and extensive ventilation facilities are mandatory. In the moist process liquid developers are required and a damp copy is obtained.

Recently, considerable effort has been directed to finding a completely dry developing process for diazotype material which would be free from objectionable limitations, which could be carried out in a simple and expedient manner without expensive equipment or skilled labor, and from which copies of good quality could be economically and efficiently obtained. Although a member of compositions for a completely dry process of development have been proposed in the past, they have been generally unsatisfactory in one or more respects. These include poor shelf-life, low image density, poor image quality, slow printing speed and objectionable odors.

French Pat. 1,249,913 discloses a thermally developable diazo composition containing a diazo compound and a coupling compound together With thiourea and urea as developers; and heat decomposable acids and acid salts as stabilizers.

While workable compositions could be formulated, these prior art heat developing systems suffered from many disadvantages, notably, very poor shelf-life (i.e., about one week), poor image density and quality, slow printing speed and background discoloration. As used herein, the term shelf-life refers to the length of time the diazo printing paper can be stored prior to use and still obtain a commercially acceptable image. Generally, the reason for the poor performance of these prior art heat developing agents was due to their slow reaction in the paper, even at room temperature, to give premature coupling. Hence, to utilize these materials in a practical manner they had to be separated from the diazonium layer by multiple coating techniques, micro-encapsulation, separate developing sheets and the like. The above disadvantages are eliminated whenthiourea and dicyandiamide are used as the thermal developer together with alkaliiacid sulfates as the stabilizer.

The use of dicyandiamide alone and in combination with thiourea as the developing agents in heat developable diazotype compositions is disclosed in US. Pat. 3,271,- 155 -While these formulations are improvements over the earlier compositions containing urea and urea derivatives, they are also burdened with chemical instability and a poor .shelf-life. For instance, the data reported in US. Pat. 3,271,155 indicates that diazo papers prepared from formulations employing dicyandiamide and thiourea as the developing agents have a shelf-life of two to three weeks. The short shelf-lives thus reported do not lend themselves to a commercially acceptable product. On the other hand, diazo compositions containing dicyandiamide and .thiourea stabilized with alkali acid sulfate salts exhibiLshelf-liVes in excess of three months and in this respect show equivalence to the conventional moist and ammonia diazo systems.

It has not been heretofore recognized that the type and proportions of diazo coupler, acid stabilizer, and thermal developer are extremely critical in obtaining good image density and long shelf-life. The instant invention for the first time presents the critical interrelationship between the various chemicals in the composition and precisely defines their limits to assure good image density, chemical stability and good shelf-life of three months or longer.

According to the present invention, dicyandiamide, thiourea and an alkali acid sulfate stabilizer are incorporated in an otherwise conventional diazotype coating composition containing a light sensitive diazonium compound and an azo dye coupler. The composition can contain colloidal silica if desired, although conventional antioxidants, stabilizers and hygroscopic agents are to be avoided. For instance, conventional hygroscopic agents such as ethylene glycol and glycerol materially reduced shelf-life. Conventional acid stabilizers such as citric acid, maleic acid, tartaric acid and boric acid reduce shelf-life from three months and longer to less than one month and are to be avoided. Only alkali metal acid sulfates such as sodium acid sulfate, potassium acid sulfate and lithium acid sulfate are suitable stabilizers if long shelf-lives are to be realized. The reason for the poor performance of the conventional organic acid stabilizers is not known, although it is suspected that the carboxyl groups somehow interact to cause precoupling on storage.

Accordingly, the diazo composition consists essentially of an aqueous solution of a diazonium salt, a diazo coupler, an alkali acid sulfate, thiourea and dicyandiamide. If desired, the diazo composition can contain finely divided or colloidal silica or alumina for improved image contrast.

The selection of the diazo sensitizer is not critical and can be any of the well known diazonium salts, particularly those diazo sensitizers in the form of double salts of zinc, cadmium and tin. The proportion of the diazonium salt is in accordance with conventional practice and is usually in the range of about 0.5 part to about 6 parts per parts of water. Included in this group of suitable compounds are: p-diazodimethylaniline zinc chloride, p-diazodiethylaniline zinc chloride, p-diazo-N-methyl-N-hydroxyethylaniline zinc chloride, p-diazoN-ethyl-N-hydroxyethylaniline zinc chloride, 4-diazo-l-morpholino benzene zinc chloride, 4-diazo-1-morpholino-2,5-diethoxybenzenecadmium chloride or zinc chloride, p-diazo-N-ethyl-otoluidine zinc chloride, p-diazo-N-diethyl-m-toluidine zinc chloride, p-diazo-N-diethyl-m-phenetidine zinc chloride, p-diazo-N-benzyl N-ethylaniline zinc chloride, p-diazo-ochloro-N-diethylaniline zinc chloride, 4-diazo-1-morpholino-2,5-dibutoxybenzene zinc chloride or sulfate, 4 -diazo- 2,5-dimethoxy-4-tolylmercaptobenzene zinc chlorlde, pdiazo-N-methyl-N-benzyl-o-phenetidine zinc chloride, 4-

diazo-1'-2,5-triethoxydiphenyl oxalate, p-diazodiphenyL' amine sulfate, and 2-diazo-l-naphthol-S-sulfonic acid.

Similarly, the azo dye coupling component can beernployed in amounts consistent with conventional diazo practice and is usually in the range of about 1 to about 6 parts of coupler per 100 parts of water. The selection of the diazo coupler is critical in that only couplers selected from the group of conventional azo dye couplers consisting of naphthalene derivatives such as the hydroxy substituted naphthalenes including 2,3-dihydroxy naphthalene, 1,8-dihydroxy naphthalene; naphthalene sulfonic acid derivatives such as 6,7-dihydroxy-2-naphthalene sodium sulfonate, 2,7-dihydroxy-3,6-naphthalene disulfonic acid, 2- hydroxy naphthalene-3,6 disulfonic acid, l-benzoylamine- 8-hydroxy naphthalene 4-sulfonic acid; and resorcinol derivatives such as diresorcyl sulfide, diresorcinol, octylresorcinol, alpha-resorcylamine Z-methyl resorcinol and monochlororesorcinol are suitable for prolonged shelflives. Conventional couplers which are not naphthalene or resorcinol derivatives such as 7-hydroxy 1,2-naphthimidazole, acetoacetanilide, and m-hydroxy phenol urea have a tendency to detract shelf-life and are unsuitable for the present purposes.

The stabilizer used is most critical for long shelf-life and good image density and quality. Only alkali metal bisulfates are suitable. The concentration employed should be sufiicient to maintain the pH below 4 and preferably less than 3 and is usually in the range of about 3 parts to about 8 parts per 100 parts of water.

Acid stabilizers conventionally used in prior art compositions include: maleic, malonic, trichloroacetic, boric, glyoxilic, sulfanilic, and tartaric. The above acids were found to seriously limit shelf-life in that they caused premature color development in the diazo printing paper. [In the case of boric acid particularly, only very small quantities, i.e., in the neighborhood of 0.1 part per 160 parts of water either alone or in combination with any other acid prevented image development almost entirely.

The thiourea is an exceedingly critical element 'irt the composition. In conventional and prior art heat developable compositions thiourea is used as a diazo stabilizer and developer and is generally used as an equivalent to 1,3,6- naphthalene-trisulfonic acid, its sodium salt and "other materials such as allyl thiourea and the like. According to the present invention the thiourea not only serves to stabilize the diazo salt but it also serves as part of the developer with the dicyandiamide as described in'detail below. Other diazo stabilizers do not appear to function in this manner.

The dicyandiamide functions as only part of the heat developer system. The necessity for a controlled ratio and limits of the amounts of thiourea and dicyandiamide were not appreciated or recognized in the prior heat developing diazo art.

It has been found that for good shelf-life and image density with a minimum of background color, the total quantity of thiourea plus dicyandiamide in the diazo type composition should not be substantially above about 15 parts per 100 parts of water because of the limited solubility of dicyandiamide in water. Also, it has been found that the amount of thiourea should not be substantially less than 3 parts per 100 parts of water nor should the composite diazo type solution contain substantially less than parts of dicyandiamide per 100 parts of water, with the total quantity of the two developing agents being not substantially above parts per 100 parts of water. These concentrations are based on a solution pickup .by the paper on a conventional air-knife diazo coater of approximately 1 quart of solution per 100 square yards of coated paper. The amount of solution applied can range from 1 pint t0 2 quarts per 100 square yards. An unex- 4 plained mechanism is apparently responsible in that an equivalent amount of either dicyandiamide or thiourea alone does not produce the desired shelf-life and image quality.

The colloidal silica is an optional material. It promotes better contrast and is well known in the prior art.

Other additives used in conventional diazotype compositions seriously detract shelf-life and are to be avoided. These include zinc chloride, diethylene glycol, stabilizers other than thiourea and alkali acid sulfates.

Various combinations of diazo composition, coupler, dicyandiamide, thiourea and alkali metal acid sulfate are possible in coating paper compositions for practicing the present invention. For instance, a separate layer of dicyandiamide can be formed as a first layer on the paper over which the diazo composition is separately coated. The diazo composition can be the first coating and the dicyandiamide composition can be used as a second coating. Alternate layers of dicyandiamide composition and diazo composition can be used such as three coatings of different or the same diazo composition. One, two, three or more diazo compositions can be used to provide different colorations or other desired visual effects. Several layers of the diazo compositions can be used with one coating of dicyandiamide composition on the bottom or top layer of the diazo composition. When a porous or permeable substrate is used, a coating of dicyandiamide composition on the reverse side of the substrate containing the diazo composition can be used.

Suitable substrates for carrying the diazo composition are glass'fiber, paper, wood, composition board and nonporous materials such as glass, metal, plastic films and the like. The most commercially significant base is, of course, paper and accordingly the examples are expressed in terms of paper.

The components of the sensitizer diazo composition are preferably incorporated in a single aqueous solution or suspension and applied in a single coating step to the base. The base can be paper or a film such as cellulose, cellulose acetate or other plastic film. In one embodiment of the present invention the various components of the sensitizing composition can be applied in two successive coatings; one containing the diazonium salt and the other containing the dicyandiamide and thiourea, the coupling component and the alkali acid sulfate stabilizer being incorporated into either of the two layers.

The base coated in accordance with this invention is developed after exposure to a radiant image pattern by heating at a temperature between -200 C. and usually between -180" C. at which temperature the dicyandiamide and thiourea are converted to basic materials. This promotes coupling of the diazonium salt with the azo dye coupling component in the non-exp0sed areas to form the azo dye image.

For best results it is preferred to prepare and coat the diazo compositions at relatively low temperatures, i.e., below 55 C. to prevent decomposition of the diazonium salts and also to prevent premature reaction of the developer. Since dicyandiamide has a limited solubility in water, in some cases it is advantageous to precoat the diazotype base paper with part of the dicyandiamide to avoid crystallization of dicyandiamide from the coating solution. It is known that image quality suffers when crystals of dicyandiamide are present in the coating solution.

Such crystallization can be avoided by precoating the base paper with part of the dicyandiamide at the size press on a conventional Fourdrinier paper machine during the manufacture of the diazotype paper base. This is readily accomplished by adding a portion of the dicyandiamide to the starch sizing solution and applying it to the paper surface on the paper machine. The remainder of the dicyandiamide is applied with the sensitizing solution. Thus the possibility of crystallization of the dicyandiamide from the sensitizing solution during the subsequent diazo coating operation is avoided since a lower concentration of dicyandiamide is present.

'It has been found that a copying paper prepared by coating a base sheet of paper with the composition of the invention is very stable, provides good shelf-life, has good image density with a very low level of background discoloration. The papers thus produced have the desirable properties of conventional commercial ammonia developed papers.

The product of the instant invention can be developed dry simply by exposure to heat. While the development step is not completely understood, it is believed that at temperatures in the range of about 120-180 C., the thiourea and the dicyandiamide co-react and decompose and raise the pH of the coating to a point where the azo dye coupler and the diazonium compound can form a dye in areas in which the diazo salt has not been exposed to actinic light. However, because the reactions which occur during development are not fully understood, it is not intended to limit the invention to any particular theory d of operation. The following compositions are coated on conventional diazo base paper using a conventional airknife coater. The coatings are applied at the rate of approximately 1 liter of solution per 100 square yards of paper surface in accordance with conventional diazo coating practice.

The following examples will illustrate the principles of the present invention although they should not be construed as limiting. All parts and percentages are by weight unless otherwise indicated. The compositions in the following examples are similar to Class 2, Subclass A-l, Style E of Government specification UU-P-221A, dated Oct. 27, 1964 and entitled Paper, Direct-Positive Sensitized (DiazotypeMoist and Dry Process).

EXAMPLE 1 A white paper base was sensitized with the following thermally developable diazo composition. This composition is a Standard Speed, Blue Line similar to Type I-B, Speed A of the Government specification mentioned above.

Parts by weight Water 100 Sodium bisulfate 6 Thiourea 3 Dicyandiamide 12 6,7-dihydroxy-2-naphthalene sodium sulfonate 5 p-Diazo-N-ethyl-N-hydroxy ethylaniline zinc chloride 2 Colloidal silica 3 Part of the paper was dried and exposed to light under a transparent original having an opaque image pattern thereon. The exposed paper was developed by passing it through a heated chamber having a temperature of about 150 C. A blue dye image was formed in the image areas of the heat developed print. The diazo paper had a speed rating of about 50 when measured according to Test 4.1.3 of the above mentioned Government specification. The maximum (image) density of a fresh sample was determined by Test Method 4.14 of the Government specification.

To demonstrate the shelf-life of the diazo paper formed in this example, the aging characteristics were determined according to Test Method 4.14.1 [Minimum (Background) Density] according to the above mentioned Government specification. These test values were compared with the Minimum (Background) Density of a fresh sample according to Test Method 4.14.1 to determine the amount of precoupling on storage.

Briefly, the maximum image density test comprises completely developing, without exposure, a fresh sample of diazo test paper. For opaque diazo papers the image density is a function of the ratio of the reflected light to the incident light. Specifically, the image density is defined as the log of the opacity where opacity is the quotient of the incident light divided by the reflected light. High image density (low percentage of reflectance) indicates a desirable dark image.

The background density test comprises completely exposing the diazo sample sheet so that all the diazo material will be entirely decomposed. The exposed sheet is then developed and the density measured. Low background density (high percentage of reflectance) indicates a desirable white background. If the background density increases on storage, precoupling or poor shelf-life stability is indicated. A background density of about 0.18 or about 66% reflectance is usually considered the maximum for commercial acceptability. The background density of a fresh diazo paper is given for comparison with the background density of a stored sample. In these examples, the samples were stored at room temperature (i.e., 70 F.) in a sealed black plastic bag for protection from the light.

The maximum density of the paper produced in Example 1 was 1.25 or about 5.6% reflectance.

The background density of a freshly prepared sample was 0.10 or about reflectance.

Upon storage for 15 days, the background was still 0.10. Upon storage for days, the background density was 0.13 or about 74.1% reflectance.

This data indicates that there was little precoupling or storage and the paper was well within the limits of commercial acceptability.

EXAMPLE 2 A white paper base was sensitized with the following thermally developable compositions to produce a Super Speed, Blue Line diazo paper similar to Type l-B, Speed C of the Government specification described above.

Parts by weight Water Sodium bisulfate 5 Thiourea 5 Dicyandiamide l0 6,7-dihydroxy Z-naphthalene sodium sulfonate 3 4-diazo-1-morpholino-2-S-diethyloxybenzene 1 Colloidal silica 3 The paper was dried and exposed to the light under a transparent original and thermally developed as in Example l. A blue dye was formed in the image areas of the heat developed print. The diazo paper had a speed rating of about by the test method of Example 1.

The aging characteristics were determined by the method of Example 1 and the following results were obtained:

Reflectance,

percent Maximum density, fresh paper 0. 95 11.2

Background density, fresh paper 0. 10 80 Background density after- 15 days storage 0. 10 8O 90 days storage 0. 12 76 This data indicates the excellent storage stability of the Super Speed, Blue Line diazo papers prepared according to the present invention.

EXAMPLE 3 A white base paper was sensitized with the following heat developable composition to produce a Standard Speed, Black Line diazo paper similar to Type l-A, Speed A of the Government specification.

Parts by weight The paper was dried and then exposed and developed as in Example 1. A black dye was formed in the image areas of the heat developed print. The diazo paper had a speed rating of about 55 by the method of Example 1.

The imaging characteristics were also determined according to the method of Example 1 with the following results.

Reflectance,

percent Maximum density, fresh paper 1. 20 6. 3

Background density, fresh paper 0.10 80 Background density after- 15 days storage 0.10 80 90 days storage 0. 16 69 This data indicates the excellent storage stability of Standard Speed Black Line diazo paper prepared according to the present invention.

EXAMPLE 4 A white base paper was sensitized with the following thermally developable composition to produce a sepia-type diazo composition similar to Type l-D, Speed N of the Government specification.

Parts by wt. Water 100 Sodium bisulfate 6 Thiourea 3 Dicyandiamide 12 Diresorcyl sulfide 4 p-Diazo N ethyl N hydroxy ethylaniline zinc chloride 3 Reflectance,

percent Maximum density, fresh paper 0.90 12. 6

Background density, fresh paper-. 0. 80 Background density after-- 15 days storage 0.10 80 90 days storage 0. 17 68 The above data demonstrates the excellent shelf-like stab1l1ty of sepia diazo paper of the present invention.

EXAMPLE 5 A white base paper was sensitized with the following thermally developable composition to produce a Standard Speed, Blue Line diazotype paper similar to Type 1-B, Speed A of the Government specification.

Parts by Wt. Water 100 Sodium bisulfate 6 Thiourea 3 Dicyandiamide 5 6,7-dihydroxy-2naphthalene sodium sulfonate 5 p-DiaZo-N-ethyl-N-hydroxy ethylaniline zinc chloride 2 Colloidal silica 3 This composition was applied to the base paper stock which had been treated with the equivalent of 7 parts of dicyandiamide with the starch sizing on the size press of a standard Fourdrinier paper making machine.

The paper was dried and then exposed and developed as in Example 1. A blue dye was formed in the image areas of the thermally developed print. The diazo paper had a speed of 50 according to the method of Example 1.

The imaging characteristics on storage were also obtained according to the test method the following results:

of Example 1 with Reflectance percent Maximum density, fresh paper 1. 25 5. 6

Background density, fresh paper 0. 10 Background density after 15 days storage 0. 10 80 days storage 0. 14 73 This data indicates the excellent storage stability of thermally developable Standard Speed, Blue Line diazo paper prepared by applying a portion of the dicyandiamide developer with the starch sizing and then applying the balance of the dicyandiamide with the diazo composition.

CONTROL p-Diazo-2,5-diethoxy 1 morpholinobenzene /2 zinc chloride gr 4.0 Sodium-2,3-dihydroxynaphthalene-6-sulfonate gr 1.4 Citric acid gr l0 Thiourea gr 5 Cyanoguanidine gr 5 Water ml Thepaper was dried and subjected to the storage stability test described in Example 1 with the following results.

Reflectance percent Maximum density, fresh paper 0.90 12. 6

Background density, fresh paper 0.10 80 Background density after l5 days storage 0.38 42 90 days storage 0. 89 1 9 This data indicates that the control composition containing citric acid as a stabilizer has a very short shelflife and some type of precoupling reaction occurs on storage. This is evident by the high background density after storage for only 15 days.

Having thus described the invention, What is claimed is:

1. A light-sensitive, thermally developable diazotype composition having a pH of less than about 4 and consisting essentially of an aqueous solution having the following weight proportions of ingredients per 100 parts by weight of water:

about 0.5 to about 6 parts of a light-sensitive diazonium salt,

about 1 to about 6 parts of a diazo coupler selected from the group of azo dye couplers consisting of hydroxy substituted naphthalene derivatives and resorcinol derivatives,

about 3 to about 8 parts of an alkali metal bisulfate salt, and

at least about 5 parts of dicyandiamide, and at least about 3 parts thiourea wherein the parts of dicyandiamide plus the parts of thiourea are not substantially in excess of 15 parts.

2. The composition of claim 1 further including colloidal silica or colloidal alumina.

3. The composition of claim 1 wherein said coupler is a naphthalene sulfonic acid derivative azo dye coupler.

4. The composition of claim 3 wherein said naphthalene sulfonic acid derivative is 6,7-dihydroxy-2-naphthalene sodium sulfonate.

5. The composition of claim 1 wherein said coupler 'is diresorcyl sulfide.

6. The composition of claim 1 wherein said alkali metal bisulfate salt is sodium acid sulfate.

7. A ther ma-lly developable diazo photoprinting sheet having a surface coating thereon, said coating consisting essentially of the following ingredients in parts by weight:

about 0.5 to about 6 parts of a light-sensitive diazonium salt;

about 1 to about 6 parts of a diazo coupler selected from the group of azo dye couplers consisting of hydroxy-substituted naphthalene derivatives and resorcinol derivatives;

about 3 to about 8 parts of an alkali metal bisulfate salt;

at least about 5 parts of dicyandiamide and at least about 3 parts of thiourea wherein the parts of dicyandiamide plus the parts of thiourea are not substantially in excess of 15 parts.

References Cited UNITED STATES PATENTS 3,199,982 8/1965 Kashiwabara 96 -49 3,271,155 9/1966 Aebi 9649X 3,353,984 11/1967 Landau 9691 X 3,360,369 12/1967 Amariti et al. 9649 X 3,389,996 6/1968 Welch 9691 3,431,109 3/1969 Bialczak et al. 9649 3,453,112 7/1969 Schaetfer 9691 3,493,374 2/1970 Roncken et al. 9649X FOREIGN PATENTS 1,016,530 1/1966 Great Britain 9649 CHARLES L. BOWERS, 1a., Primary Examiner US. Cl. X.R. 

